1. Field of the Invention
The invention relates to a method of detecting the frequency position in digital communication transmissions utilizing a preamble of a repeatedly transmitted pseudo random number sequence. Such a known method is disclosed in German Patent Document DE-C2-2,822,874.
2. Description of Related Art
The known method is used, in particular, for high-speed serial telegraphy in the short-wave domain. Heavy linear distortions, phase shifts and frequency offsets of the signals occur on the transmission path. The received signal is usually a mixture of several "echoes". Therefore, all these effects must be detected in the receiver and must be utilized to correct the signal. The known method employs a repeatedly transmitted PN sequence (PN=pseudo noise, that is a pseudo noise sequence) of, preferably, 31 bits in length as a preamble to the individual data blocks. At the receiving end, the preamble is compressed in a quadrature procedure by transverse filters. Preamble and data blocks are processed separately in that filter settings are obtained from an examination of the preamble, and they are employed in the data path to correct the echoes.
FIG. 1 shows the known arrangement for ("quadrature") demodulation of the transmitted carrier-added signal at the receiving end. The arrangement corresponds to that of a coherent receiver. Correction takes place in the base band. Two channels--a sine channel and a cosine channel--are necessary for the "quadrature" demodulation, in which the linearly distorted carrier-added received signal is multiplicatively mixed with a carrier signal made available by a local oscillator, directly in one case and after a carrier phase shift of 90.degree. in the other case. Since thus two channels (the sine channel and the cosine channel) must be processed in the echo correction processor, the pulse response in both channels must also be determined. In order to be able to utilize all of the energy of the useful signal available in a channel, the sine channel and the cosine channel each include a so-called matched filter M. F. which is inverse in time relative to the respective channel and is configured as a transverse filter arrangement. The weights of the transverse filters correspond to the time-inverse pulse response of the two channels. Due to the use of these matched filters, non-linear squaring is avoided (since the method is intended to operate in a linear manner). The transverse filter outputs in the matched filter are combined by adders so that two output signals result which are fed to a reciprocal filter R. F. (quadrature lowpass equalizer). The corrected data are available at the output of the reciprocal filter.
Instead of the double-sideband modulation employed here, it is also possible to employ residual sideband modulation. In this modulation process, the evaluation of the signals in the reciprocal filter can be limited to the output signal of the cosine branch of the matched filter, that is, to the real component of the complex signal.
For this case, the transverse filter arrangement in the matched filter can be simplified accordingly.
The frequency difference (frequency deviation) between the received signal and the receiver may reach up to 60 Hz since the frequency accuracy standard in simple transmitting and receiving devices lies at about 10.sup.-6 and the frequency range extends up to 30 MHz.
With a modulation rate of 3000 Bd, a 31 bit PN sequence lasts T.sub.PN =31/V.sub.T =10.3 msec, where T.sub.PN is the duration of the PN sequence and V.sub.T is the modulation rate (baud rate). With simple correlation techniques, that is, pulse compres-sion with the aid of a transverse filter whose weightings (+ and - ones) are inverse in time with respect to the PN sequence (matched filter in the time domain, sine and cosine channel) it is possible to operate with a frequency dif-ference that corresponds to a 90.degree. shift for the length of the sequence. For 10 msec, this is approximately 25 Hz. If the frequency difference is greater, the so-called ambiguity function results in a considerable reduction of the excess compression at the output of the correlation filter.